Skip undefined values for props

[Kerumen]

First of all, thanks for the fork! I enjoy using it instead of styled-system.

I have an issue when I want to display my component in Storybook because it sets all props of the component to undefined (I guess it's needed for the UI, to display the controls? I'm not sure).

Anyway, I want to render a Box (<Box paddingX={2} />) but Storybook sets paddingLeft, paddingRight, etc to undefined which overwrites the paddingX. Is it possible to ignore undefined values when parsing the props?

A fix was proposed to styled-system (PR here) but I don't have enough time to dig into your code and propose the same fix.